Announcement (2017-05-07): www.ruby-forum.com is now read-only since I
unfortunately do not have the time to support and maintain the forum any
more. Please see rubyonrails.org/community and ruby-lang.org/en/community
for other Rails- und Ruby-related community platforms.

Hi,
thanks to Marshalls help, I figured out, that my problem was caused by a
badly
chosen name of a variable in the validates method of my interface model.
Originally I called it @device, after renaming it the error was gone.
Thanks
Peter